This invention relates to a display device with a shadowmask CRT (cathode ray tube), and more particularly, to a method of electron-beam scanning.
A plurality of perforations are formed in the shadowmask in a regular arrangement in the X-axis direction (i.e., horizontal direction) and in the Y-axis direction (i.e., vertical direction). Scanning lines produced when the electron beam impinges through the perforations of the shadowmask onto a light-emitting section are arranged at a substantially equal pitch in the Y-axis direction. Thus, Moire pattern, which is an interference pattern produced by superimposing two types of regular pattern, appears on the phosphor screen.
The principle by which Moire pattern is produced may be explained as follows. When light-emitting sections 20 are disposed as shown in FIG. 1A (in this case the perforations of the shadowmask are disposed in the same manner), the average luminous efficiency T.sub.A (Y) of two neighboring columns of the light-emitting sections 20 that extend in the Y-axis direction (the two neighboring columns of the light-emitting sections are designated by 20a in FIG. 1A) will be as shown in FIG. 1B. Here, P.sub.A designates the pitch in the Y-axis direction of non-light-emitting sections 21 on the phosphor screen (which is substantially equal to half the pitch at which bridges 11 are disposed, as shown in FIG. 3B), and is known as "effective pitch." The average luminous efficiency T.sub.A (Y) is expressed by the following equation using a Fourier series, ##EQU1##
On the other hand, the density of the electron beam that excites the phosphor screen, that is to say, the excitation density T.sub.B (Y) exhibits periodicity in the Y-axis direction, as shown in FIG. 2. Here, P.sub.B is an interval in the Y-axis direction between neighboring horizontal scanning lines extending in the X-axis direction. The excitation density T.sub.B (Y) is expressed by the following equation (2) using a Fourier series ##EQU2##
Thus, luminance distribution L(Y), which changes according to the position in the Y-axis direction, may be expressed as the product of T.sub.A (Y) and T.sub.B (Y) as follows: ##EQU3##
In Equation (3), the second term represents the mosaic pattern of the light-emitting sections 20, the third term represents the scanning line pattern, and the fourth term may be rewritten as follows: ##EQU4##
Since the first term of Equation (4) is a periodic: function of pitch, which is smaller than pitch of either the mosaic pattern of the light-emitting sections 20 or the scanning line pattern, it has no effect on picture quality. But the second term of Equation (4), known as Moire term, is a periodic function that may have an extremely large spatial function (pitch), where the pitch is {P.sub.A P.sub.B /(mP.sub.B -nP.sub.A)}, and the amplitude is (A.sub.m B.sub.n /2), so there is the possibility that large Moire pattern is clearly visible to the naked eye.
One of the conditions in which Moire pattern is conspicuous occurs when the pitch {P.sub.A P.sub.B /(mP.sub.B -nP.sub.A)} becomes large, that is to say, when mP.sub.A and nP.sub.B are substantially equal. Thus, the scanning line pitch P.sub.B and effective pitch P.sub.A are normally selected in such a way that mP.sub.A does not approach nP.sub.B. Again, generally speaking, the value of A.sub.m B.sub.n is particularly large when m+n.ltoreq.5. In the case of display devices for computers, however, scanning line pitch P.sub.B is not necessarily constant, so that it is difficult to assure that mP.sub.B and nP.sub.A do not approach.
U.S. Pat. No. 4,887,010 discloses a proposal by the present applicant to overcome these problems by displacing the position of the scanning lines slightly in the Y-axis direction. This method, however, gives rise to separate problems in that scanning line pitch is made uneven, resulting in degradation in picture quality when evaluated on criteria other than Moire pattern, and in requiring a current source having an extremely wide frequency range.